4a. Inhalant Anesthetics Flashcards
What are some inhalant drugs that were historically used and are now currently used?
Hx: diethyl ether (Vomiting and explosive), chloroform (vomiting, nausea), halothan - no longer available
Current: isoflurance, sevoflurane
How is inhalants delivered?
liquid at room temperatore, stored in a vaporizer
vaporizer pressurized drug > turns into gas as a specific partial pressure
a set amount of anesthetic gas mixes with oxygen > delivers to patient
How are inhalant drugs absorpbed?
drug delivered into lungs when patient breathes in anesthetic gas mixed w/ O2
Drug enters alveolar sacs
Conc of drug in alveolar sac is higher than conc of drug in plasma
Drug diffuses across alveoli along conc gradient > enters circulation
How is inhalant drugs distributed?
Once drug enters plasma, is rapidly distributed to brain because drug is very lipid soluble, brain high in fat, brain receives lots of blood flow, drug moves along steep conc gradient: highest conc in alveoli > blood>brain, so long as drug is being delivered to lungs, will maintain brain lvls. drug active so long as in brain
not as rapid as injectable anesthetics
How is inhalant drugs metabolized?
They are <0.1% metabolized by drug. Essentially, it is not metabolized
Drugs that require liver metabolism have an extended “hangover” effect
Benefits: pharmacokinetics are unaffected by liver dz
How is inhalant anesthetics eliminated?
99.99% eliminated via lungs in active form
when gas is turned off, conc gradient reverses - higher conc in brain > rapidly enters blood > diffuses across alveoli into lungs > exhaled
As drug leaves brain, patient wakes up
How might we be able to speed up eliminating inhalant anesthetics?
can inc rate of elim by inc the conc gradient btw brain and lung/outside
1. Flushing circuit (removing drug from the lungs, mask/ETT, tubing)
2. Giving more 100% O2
How can we summarize inhalant anesthetic drug movement?
Diffusion rate controlled by conc gradient btw alveolus and blood
During induction, conc gradient highest in alveoli, lower in blood and lowest in brain, so drug move rapidly from alveoli > blood > brain
When anesthetic machine off, gradient reverses so brain > blood > alveoli
Maintenance is dependant on sufficient quantities of anesthetic delivered to lungs
takes time to reach therapeutic lvls in brain; but elimination very rapid
What are the advantages of inhalants?
- rapid elim thru lungs
- easy + fast to alter amount of drug in brain by delivering more/less into lungs. Easy to adjust depth of anesthesia
- Good muscle relaxation
- Very rapid recovery
- Can us in patients with liver/renal dz
- Patient is intubated and 100% O2 is available int he event resp depression or arrest
What are the disadvantages of inhalants?
- takes a long time to induce
- expensive equipment required w/ train personnel
- NO analgesia
- Hypotension (severe vasodilation) and moderate bradycardia
- Hypothermia - related to temp of O and heat loss thru vasodilation
What is the precautions and adverse effects of inhalant anesthetics
- dose-dependant, reversible CNS depression
- Dec HR, cardiac output
- Dec RR and tidal volume (always dec under GA. Goal to min. change
- Vasodilation!! w/ 2nd hypoperfusion
- Hypothermia - cold 100% O, vasodilation, lack of shivering
- Can cause renal damage due to dec BP - watch old, renal patients, patients on drugs that affect kidneys
What are important points to know if we are going to induce with inhalant anesthetics
never preferred. Acceptable in cats, small dogs, exotics bc cannot find vein bc fractious. Duration of GA is required is much shorter than what injectable anesthetics provide
Ideally patient also has premed
takes longer than injectable anesthetics - requires time to achieve effective levels in brain, longer transition through stage 1-2 are unpleasant for patient
req very high doses - inc risk of adverse effects, especially vasodilation
not indicated in large animals
If an inhalant anesthetic is not done through an ETT, how else can it be done?
Through chamber or mask
space of mask/chamber also needs to fill with a certain amount of drug, before drug conc gradient is high enough to move drug into blood
chamber induction takes longer and can be very stressful
Why is inhalant anesthetics the preferred maintenance anesthetic?
1 choice for all species given current techniques
easy to maintain in therapeutic range for long periods
can rapidly adjust depth of consciousness; can rapidly respond if patient is too light or too deep
faster elimination and recover than any of the injectables
Reminder: always keep anesthetic time as short as possible. Longer anesthetic times have an increased risk of complications, and have longer recovery times
How is the recovery with inhalants?
preferred bc of its smooth and rapid recovery
drug is almost entirely eliminated by lungs by breathing out (does not req waiting for liver metabolism and no redistribution to fat)
can accelerate rate of elimination by providing the lungs with more oxygen or getting drug out of lungs faster (Flushing the system)
What are the 3 chemical properties of inhalant anesthetics
- vapor pressure - how readily the drug evaporates, determins how it is delivered
- Blood-gas partition co-efficient - affects how rapidly we can increase or decrease drug lvls in the body
- Minimum alveolar conc (MAC)
used to calculate drug dose
What is vapor pressure?
A measure of the ability to evaporate under normal atmospheric pressure - remember that molecules enter the gas phase more readily under low pressures
determines how a drug is to be delivered - determines type of precision vaporizer required, precision vaporizers are canisters with a regulated internal pressure
What is a precision vaporizor?
a canister with a regulated internal pressure
What are low vapor pressure drugs?
These drugs DO NOT evaporate readily
there is very little of the drug that goes into gas form on its own; this limits the amount of drug that mixes with oxygen
safe to give with a non-precision vaporizer because a minimal amount of drug will be in gas form at atmospheric pressure
What is high pressure drugs
drugs with high vapor pressure evaporate readily from liquid to gas
at atmospheric pressure, these drugs like to be present in gas form so likely to get a higher percent of drug mixed w/ oxygen
Must be given by precision vaporizer - precision vaporizers have internal pressures; this limits the amount of drug present in gas form, limits amount of anesthetic agent
Which drugs do we use are considered high vs low pressure?
All are high pressure drugs
Desflurane > halothan > isoflurane and sevoflurane in order of highest to least highest pressure
Give a short example with isoflurance and how might it being a high vapor pressure drug could be an issue
Isoflurance has a high vapor pressure
at 20C at atmospheric pressure, isoflurance and O2 would stabilize at a ratio of 31.5 iso to 69.5 O2
Problem is 31.5% iso is a toxic dose
Precision vaporizer is a high pressure chamber that inc local pressure s less iso in gas form (goal is 0-5)
Can you interchange precision vaporizors between drugs?
Nope, except 1. Can use halothan for iso bc the pressure btw them is very similar
Extreme changes in temps will alter accuracy of precision vaporizor
What is a blood-gase partition co-efficient
a measure of how well the inhalant anesthetic dissolves into blood - amount of drug in blood relative to amount of drugs in the lungs
Is associated with speed of induction, speed of recovery and how rapidly one can change anesthetic depth
What happens with high blood gas co efficient?
older inhalant anesthetics
req lg amount of drug to build up in the blood relative to lungs to have an effect
slower induction, slower recovery
tends to be more tissue accumulation of drug (w/ long anesthetics procedures) which can inc rate and duration of “anesthetic hangovers”
How does low blood gas co efficient affect the body?
new drugs
does not require a lot of drug in blood (relative to in the lungs) to have an effect
drug also moves from lungs to blood and back to lungs quickly
fast induction, faster recovery; can more rapidly alter dept of anesthesia for less tissue accumulation and less residual effects
What does the blood gas co-efficient determine?
the clinical use/effect of the drug
1. drug w/ low BGCE are preferable for mask or chamber induction
2. Drug w/ low BGCE have greatest ability to alter depth of anes.
3. Drug w/ high BGCE have longer recovery times and patients may have anesthetic hangovers
What is MAC?
sort of like ED50
the lowest conc of an agest to prevent a response to surgical stimulation in 50% of patients (to obtain stage 2 Plane 2 if ONLY using inhalant (no other drugs)
A measure o fpotency of drug
High MAC = less potent
Low MAX = more potent
How do we dose anesthetic gas?
dose measured in %
if patient inhales 100% gase, % of this total amount if made up of the anesthetic gas
dose is diff for every drug/species
dose vary’s btw patients
Not affected by weight - bc they don’t “fill” the body and go to brain pretty quick
What is the MAC for iso in dogs, cats and horses?
Dog: 1.3
Cat: 1.6
Horse: 1.3
What is the MAC of sevoflurane for dogs?
2.4
What is the difference btw low MAC vs high MAC?
Drugs w/ low MAC: more potent, less drug to achieve anes., more likely to OD, harder to make fine changes in anes. depth, older drugs have lower MACs
Drugs w/ higher MAC: less potent, takes more drug to achieve anes. easier to make fine changes to anes. depth, newer drugs have higher MACs
Why is it important to know MAC?
MAC is used to determine dose of an inhalant anesthetic req for the average patient - giving MAC dose should provide stage 3, p. 2 in 50% of patients
For dosing using balanced anesthetic protocols, what might 1x, 1.5x and 2x MAC provide? What should we start with?
1x MAC = light anes. Plane 1
1.5x MAC = mod or surgical plane 2
2x MAC - deep anes. plane 3
Start around 1.5x MAC and adjust dose according to individual patient and depth of anes
Give an example of using MAC to determine dose for a dog
MAC for iso in a dog is 1.3%
1xMAC = 1x 1.3 = 1.3% to give stage 3 plane 1
1.5 x 1.3 = 1.95%, set @ 2 should give stage 3 plane 2
2 x 1.3% = 2.6%. Most will be too deep at 2.6%
How might MAC determination change if we use a very strong pre-med?
The acceptable starting and maintenance dose of gas may change
May only need 0.5% - 1% iso to maintain a surgical plane anesthesia
When we mask a cat, we can start by giving them 5% iso. Why?
because we want a high conc gradient and get through stage 1-2 as quick as possible
If we leave the dose as 5%, we will overdose them
Once induced, we can turn it down to 2.4% or 2.5% iso
What are some factors that may alter dose?
very old, youung
pre-existing CV or resp dz
trauma, dehydration, shock, other illness
body temp, obesity, pregnancy
if combined w/ other drugs (certain premeds, and injectable anesthetics)
Can you summarize the chemical properties of inhalant anesthetics?
- Vapor pressure - how readily drug enters gas phase, drugs that enter gas state more readily require a precision vaporizer to prevent OD, vaporizer controls amount of drug delivered and max delivered
- BGCE - drugs w/ lower co-eff are preferred, have faster induction and recovery, better adjustment, how readily the drug dissolves in blood
- MAC - measure of drug potency, can use to calculate dose of gas required
What is isoflurane?
Standard to maintenance anes.
Can be used for mask or chamber induction
Pungent odor - good for detecting leaks
good muscle relaxation, no analgesia
>99.8 eliminated from lung, 0.2% liver metabolism
What are the chemical properties of iso?
High vapor pressure reqs precision vaporizer
Relatively low BGCE - faster induction and recovery than halothane; not as fast as sevo or des
Patient responds in seconds to changes in dose
What is the MAC for iso? What is the starting maintenance setting for each species
1.3% (D), 1.6%(C), 1.3%(Eq)
Maintenance
Dog: 1.5x1.3 = 1.95
Cat: 1.5x1.6=2.4
Eq: 1.5 x 1.3 = 1.95
What are the adverse effects of iso?
vasodilation
Hypothermia
Decreased co2 drive
Malignant hyperthermia
Why is vasodilation an adverse effect for iso?
Always occurs, worse than Ace. 2degree hypotension can cause delayed post-op renal damage
Dose dependant CNS depression, dec HR, RR, cardiac output, tidal volume
Why is hypothermia an adverse side effect of ise?
due to vasodilation, suppression of hypothalamus, lack of shivering and delivery via cold oxygen
irritating to mucus membranes - can cause patients to fight mask induction
Can have very slight hangover effect due portion that requires liver metabolism
Why is depression of the co2 drive an adverse effect for iso?
normally, high lvls of CO2 are the brain’s signal to make the body breath. This is co2 drive, without it, you actually stop breathing until those lvls inc
This signal can be lost with iso
2x-3x MAC can cause resp arrest in most patients (3-4.5x in dogs)
Iso is a risk to pregnant staff, why?
A potential abortifactant
rapidly crosses placenta
causal association w/ spontaneous abortion (not proven); may also be related to decreased memory
Pregnant staff should not be around when mask inducing or chamber induction
allow fresh air to recirculate into room before entering
How does mask induction with iso proceed?
for mask/chamber induction, will typically use the highest dose setting on vaporizer - 5% for iso (bc get to stage 3 faster)
As soon as they are induced, MUST drop to maintenance dose (~2-2.5% iso) or patient will risk anesthetic overdose
Animal will fight induction because irritating to MM, distinct odor or slow progression thru stage 1-2
What is sevoflurane?
2nd most common, newer, $$$
Very similar to iso
almost entirely exhaled by lungs, good muscle relaxation, no analgesia
What are the chemical properties of sevo?
high vapor pressure - requires precision vaporisor
BGCE lower than iso - less soluble in blood = drug reaches brain faster, faster induction and recovery, minimal drug dissolves in blood; therefore, drug almost entirely eliminated by lungs
MAC: 2.4%(d), 2.6%(c), 2.3%(eq)
What are the adverse effects of sevo
CNS depression
CV depression: vasodilation less than iso, dec HR and cardia output
Resp depression: dec RR and tidal volume - does not turn off c02 drive the way iso does, patients breathe better of sevo than iso
What are the advantages of using sevo over iso?
- faster induction/recovery
- High BGCE = more smaller adjustments to dose and fine tune depth of anes. can be made
Important for horses, range for sevo is 2.5-4, Iso is 1.5-2.3
No smell, not irritating to MM (preferred over iso for mask)
does NOT turn off co2drive
Sevo is expensive!
What is nitrous oxide?
N20, laughing gas
use at 33% o2 + 66% n2o
33% absolute minimum amount of O2 required for patient under GA
Can be part of balanced anesthesia
1. speeds induction/recover
2. analgesia
3. significantly decreases dose of other inhalant anesthetics (up to 30% dec in MAC)
What is important to do once we turn of n2o? Why does this happen?
Patient MUST be left of 100% o2 after turning n2o off to prevent diffusion hypoxia
As soon as n2o turned off, drug flows out of body and accumulates in lungs. For brief period, there is actually going to be high % of nitrous in lungs
Room air is only 21% O and is insufficient to meet demands as patient is recovering
Some contraindications
